OLED technology incorporates organic luminescent materials that, when sandwiched between electrodes and subjected to a DC electric current, produce intense light of a variety of colors. These OLED structures can be combined into the picture elements, or pixels, that comprise a display. OLEDs are also useful in a variety of applications as discrete light-emitting devices or as the active element of light-emitting arrays or displays, such as flat-panel displays in watches, telephones, laptop computers, pagers, cellular phones, calculators, and the like. To date, the use of light-emitting arrays or displays has been largely limited to small-screen applications such as those mentioned above.
The market is now, however, demanding larger displays with the flexibility to customize display sizes. For example, advertisers use standard sizes for marketing materials; however, those sizes differ based on location. Therefore, a standard display size for the United Kingdom differs from that of Canada or Australia. Additionally, advertisers at trade shows need bright, eye-catching, flexible systems that are easily portable and easy to assemble/disassemble. Still another rising market for customizable large display systems is the control room industry, where maximum display quantity, quality, and viewing angles are critical. Demands for large-screen display applications possessing higher quality and higher light output has led the industry to turn to alternative display technologies that replace older LED and liquid crystal displays (LCDs). For example, LCDs fail to provide the bright, high light output, larger viewing angles, and high resolution and speed requirements that the large-screen display market demands. By contrast, OLED technology promises bright, vivid colors in high resolution and at wider viewing angles. However, the use of OLED technology in large-screen display applications, such as outdoor or indoor stadium displays, large marketing advertisement displays, and mass-public informational displays, is only beginning to emerge.
Several technical challenges exist relating to the use of OLED technology in large-screen applications. Presently, in the case of a small-screen application in which the display typically consists of a single OLED display panel, OLEDs age more or less uniformly. Thus, when the light output is no longer suitable, the entire display is replaced. However, for large-screen applications, where the display may consist of a set of tiled OLED display panels, there is the possibility that one OLED display will age at a faster rate than another. Typically, when a tiled OLED display is manufactured, it is calibrated for a uniform image. Age differences occur, for example, due to the varying ON times (i.e., the amount of time that the OLED has been active) of the individual OLEDs and due to temperature variations within a given OLED display area. In addition, age differences in the overall display may exist due to replacement of an older tile with a newer tile. Tiles may be replaced when a module is damaged or found to be defective. The result of using the display's modularity to replace individual tiles is non-uniformity of the overall display, as the light output of newer replacements may be inconsistent with older existing OLED modules.
An example of a method to correct non-uniformities in an initially calibrated OLED display device is described in WO 01/63587, entitled, “Method and apparatus for calibrating display devices and automatically compensating for loss in their efficiency over time.” The '587 patent application describes a method of OLED compensation for loss of uniformity of the display output of a display including organic light-emitting devices (OLED) due to aging. Since the decay of emitted light follows an exponential law, change in light output due to aging can be predicted by accumulating (i.e., performing numeric integration) the driving current for each individual pixel during an elapsed time. Then, based on such predicted change, the driving current can be adjusted for each pixel to compensate the decay.
Another example of a method to correct non-uniformities in an initially calibrated OLED display is described in WO 99/41732, entitled, “Tiled electronic display structure”. The '732 patent application describes a method of compensation for loss in brightness due to aging of OLEDs in a display tile. Two methods for electronic compensation are described: integrating current during an elapsed time and comparing it to a characteristic curve, and measuring the change in voltage due to aging, which change in voltage is proportional to a change in brightness of the OLEDs. Both methods allow to adjust the drive current of the OLEDs, thus automatically maintaining a constant brightness without manual adjustments.
Although the compensation techniques described in the '587 and '732 patent applications provide a satisfactory means of compensation for many OLED applications, it does not adequately address the concerns of a display composed of many discrete tiles of various ages that are subjected to different aging conditions.
It is therefore an object of the invention to provide a method and device for optimizing uniformity in the light output and color over the lifetime of devices in which light output may deteriorate, and colors may shift due to aging, more particularly, but not limited to, an OLED display device, particularly a tiled OLED display device.
It is a further object of the present invention to increase the lifetime of an OLED display, more particularly, but not limited to, a tiled OLED display device by maintaining uniformity in the light output and color over a longer lifetime of the device.